Partially aromatic polyamides with terephthalamide- or isophthalamide units have been known in principle for several decades already; however, they did not become commercially available since the seventies on account of their demanding production methods. Methods and apparatuses for the production of high-melting, partially crystalline polyterephthalamides have even not been available until the middle of the eighties (e.g. the so-called "aerosol process" by AMOCO as described in U.S. Pat. No. 4,831,108).
Because the pure high-temperature polyamides (homopolymers) are very difficult to manage on account of their high melting point, usually, copolyamides derived therefrom with reduced melting point (or even amorphous ones, depending on the molar ratio) are produced such as e.g. PA 6T/6I or PA 6T/66 or PA 6T/6, which are only partially aromatic (as regards the amount of dicarboxylic acid) and which, however, also offer interesting possibilities of industrial use.
Theoretically, a plurality of different, partially aromatic copolyamides from aromatic dicarboxylic acids, diamines as well as lactams (respectively .omega.-aminoacids) are available. However, all previously known production methods share the common feature that the above-mentioned components are used as such in pure, monomeric form as raw material and that dicarboxylic acid(s) and diamine(s) are purposefully converted in a solution container connected in front of the polycondensation reactor at first in approximately equimolar amounts to aqueous, optionally lactam-containing "nylon" saline solutions.
Apart from the above paragraph and at first apparently without a connection to the end product striven for, there is a known problem in another area of polymer use: In the bottling of beverages, especially for carbonated soft drinks, user-friendly polyester bottles have largely displaced glass bottles. The problem of recycling arises for the large number of non-returnable bottles and also for worn-out returnable bottles because the pile of household waste should not be additionally enlarged by polyester bottles and on the other hand polyester bottles constitute in principle a valuable raw material. Various concepts for the collection and reuse of polyester bottles have already been developed and partially introduced such as is presented, e.g., in the lecture of H.-W. Blumschein: "Chance or Risk for PET-Packaging, -Recycling-, How to Close the Loop in Western Europe" (Davos Recycle '92, International Forum and Exposition, Apr. 7-10, 1992). In addition to the purely mechanical comminution, purification and reuse (flakes or regranulate for fibers, etc.) this lecture also describes chemical recycling methods such as e.g. methanolysis or glycolysis, which comprise the complete degradation of the polymer down to the monomers. Because the monomers may be reused to produce polyester for grocery packagings it is possible to close the circuit in this manner; however, this is not the most economical way to reuse material. In particular, the purification of the recycling monomers is expensive in this method. An example of this is the method described in EP 0,497,662 A1 for the production of pure terephthalic acid from PET waste. This method comprises the depolymerization of PET chips with pure sodium hydroxide, the dissolving of sodium terephthalate in water with subsequent purification in a special activated-carbon--adsorption/regeneration process, precipitation of terephthalic acid with sulfuric acid as well as filtering off, washing and drying the pure terephthalic acid. In addition to the high expense the method has two serious disadvantages: A large amount of sodium sulfate, which must be disposed of, as byproduct and the resulting terephthalic acid is very fine-grained on account of the precipitation at a relatively low temperature in comparison to commercial terephthalic acid and is therefore difficult to handle.
DE 32 21 341 C2 describes a depolymerization method of PET waste with neutral hydrolysis and simultaneous activated carbon purification. The disadvantage of this method resides in the high pressure necessary (high-pressure container) and the relatively long residence times.
In order to circumvent the expensive detour via the decomposition into monomers, suggestions have been made for preparing polyester waste by suitable methods directly for reuse. EP 0,483,665 A2 describes a method in which the PET waste is melted, filtered and in the last step after-condensed either in melt phase or solid phase to a higher (or original) molecular-weight level. Such higher-valency higher-analysis! PET regranulate can be used after extrusion together with original raw material in the middle layer of foils.
According to a report in Plastic Focus, Vol. 26, No. 19, Sep. 26, 1994, a similar, improved method ("super cleaning") apparently succeeded in achieving a recycling PET quality which can be used in direct contact with groceries. In spite of this breakthrough one must realize that the circuit can never by completely closed in this way, that is, to approximately 100% and that "virgin" granulate must always be admixed for the high requirements of purity placed on grocery and beverage packagings because in organic material like PET degradation reactions always occur at the melt temperatures which impair the quality. Therefore, for a part of the recycling polyester further possibilities of use should be sought in addition to the use for other polyester applications.
A first attempt to produce other products from polyester waste is described in H. G. Zengel, M. J. Bergfeld: A new process for the production of p-phenylenediamine alternatively from polyester waste, terephthalic ester, or terephthalic acid; Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 3, 1976, pp. 186-189. As the title already states, p-phenylenediamine can be produced from PET waste which p-phenylenediamine can be used as raw material to produce e.g. azo dyes, diisocyanates or aromatic polyamides. The process, carried out in suspension or solution, consists of the three steps PET degradation with ammonia to terephthaldiamide, the reaction with chlorine to terephthaldichloroamide as well as of a Hofmann rearrangement with sodium hydroxide solution to diamine (with subsequent purification). However, this expensive method with not quite harmless reagents still does not supply an end product but rather a monomeric intermediate product.